In conventional rotary drilling, a drill bit(s) is mounted on the end of a drill string, and a mixture of liquid and additives (drilling fluid or “mud”) is pumped down the inside of the drill string and exits as fluid jets at the bit nozzles to cool and clean the bit, and flush the cuttings to the surface as the drill bit(s) grinds away at the rock formation. Drilling efficiency is generally governed by whether the rock cuttings are effectively removed by hydraulic forces of the drilling fluid jets.
Rotary drilling generally involves the application of axial force known as Weight-on-Bit (“WOB”, the amount of downward force exerted on the drill bit), and rotary torque known as Torque-on-Bit (“TOB”, the amount of rotational or turning force exerted on the drill bit), to push and rotate the drill bit to generate the cuttings. Unfortunately, it is well known that with increased depth, there is an increase in bottom hole pressure (“BHP”) which holds the drill cuttings against the cutter face and the bottom of the hole, thereby impeding their efficient removal. This results in a reduction in the rate of penetration (“ROP”) of the drill bit, and potentially increased well costs.
Currently, there are drilling tools which provide axial compliance in the drill string for the purpose of damping drill string vibration and shock in an attempt to overcome some vibration-related problems. These are typically called “shock subs” and are provided by several drilling technology companies. There are other drilling tools which provide axial force generation at the bit, but not in combination with axial compliance. There is no other drilling technology that specifically targets providing axial displacements at the drill bit through the use of axial compliance to allow the cuttings that are held down by the BHP to be displaced and removed more easily, and result in an increase in the drilling ROP. What is therefore needed is a novel, industrially viable, vibration assisted rotary drilling (VARD) tool that can increase ROP and drilling efficiency.